CHM242H5 Chapter Notes - Chapter 15: Catenation, Sigma Bond, Hydrocarbon

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Published on 2 Dec 2013
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Chapter 15: Organic chemistry 4/8/2013 6:55:00 AM
Most organic molecules have more complex structures than most
inorganic molecules
Electron configuration, electronegativity and covalent bonding:
Carbon’s formation of covalent bonds rather than ionic bonds is the
result of its electron configuration and its electronegativity value
Carbon’s ground state configuration is [He] 2s2 2p2
The loss of four electrons to form the C4+
cation requires energy equal to the sum If the IE1 through IE4, the
gain of four electrons to form the C4- anion requires the sum of EA1
through EA4, the last three steps of which are endothermic
Lying at the center of period 2, carbon has an electronegativity,
EN=2.5, that is midway between that of the most metallic element
and the most non-metallic element
Bond properties, catenation and molecular shape:
The number and strength of carbon’s bond lead to its property of
catenation
Catenation is the ability to bond to itself
Carbon’s small size and ability to form hybrid orbitals and multiple
bonds increase the number of different molecules by affecting
molecular shape
The small size of carbon allows close approach to another atom and
thus greater orbital overlap, so forming short strong bonds
The C-C bond is short enough to allow side to side overlap of half-
filled unhybridized p orbitals and the formation of multiple bonds,
which restrict rotation of attached groups
Molecular stability
Although silicon and several other elements also catenate, none can
form chains as stable as those of carbon
Atomic size and bond strength: as atomic size increases down the
group 4, bonds between identical atoms become longer and weaker
Relative enthalpies of reaction: a C-C and a C-Cl have nearly the
same energy so relatively little heat is released when a C chain
reacts and one bond replaces the other
Orbitals available for reaction: unlike C, Si has low-energy d orbitals
that can be attacked by lone pairs of incoming reactants
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The chemical diversity of organic compounds is founded on atomic and
bonding behavior and is due to three related factors (Bonding to
heteroatoms, electron density and reactivity and importance of functional
groups
Heteroatoms are atoms other than C and H
Many compounds contain heteroatoms, most common of which are N and
O
Most reactions starts when a region of high electron density on one
molecule meets a region of low electron density on the other
These regions may be due to the presence of multiple bond properties of
carbon-heteroatom bonds
15.2
The carbon-carbon bond form the skeleton
Carbon skeleton the longest continual chain is the backbone and any
branches are the limbs
Hydrocarbons are the simplest type of organic compound containing only
C and H
Groups joined by a single (sigma) bond are free to rotate
As the total number of C atoms increases the number of different
arrangements increases as well
The arrangement of C atoms determines the skeleton so a straight chain
and bent chain represent the same skeleton
Hydrocarbons can be classified into four main groups
Alkanes:
A hydrocarbon that only contains a single bond
CnH2n+2
The alkanes comprise a homologous series, differing be a CH2
In an alkane, each C is hybridized
Alkanes are saturated hydrocarbons
The expanded formula shows each atom and bond
A cyclic hydrocarbon contains one or more rings in structure
When a straight chain alkane forms a ring, two H atoms are lost
CnH2n
Cycloalkanes are non-planar because of the need to minimize
electron repulsion between adjacent H atoms
As a result, orbital overlap of adjacent C atoms is maximized
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Compounds with the sane molecular formula but different
properties are called isomers
Those with different arrangements of bonded atoms are structural
(or constitutional) isomers
Alkanes are nearly non-polar, their physical properties are
determined by dispersion forces
The more spherical members boils lower than the more elongated
one, because a spherical shape leads to less intermolecular contact
and thus lower total dispersion forces that does an elongated shape
The greater the intermolecular contact the stronger the dispersion
forces and the higher the boiling point
Stereoisomers are molecules with the same arrangement of atoms
but different orientation of groups in space.
Optical isomerism is one type of stereoisomerism
Optical isomerism; when two objects are mirror images of each
other and cannot be superimposed
Optical isomers are also called enantiomers
An asymmetric molecule is called chiral
An organic molecule is chiral if it contains a carbon atom that is
bonded to four different groups
Properties of optical isomers: optical isomers are identical in all but
two aspects
o In their physical properties: optical isomers differ only in the
direction that each isomer rotates the plane of polarized light
o A polarimeter is used to measure the angle that the plane
rotated
o An optical isomer is optically active because it rotates the
plane of this polarized light
o The dextrorotatory isomer rotates the plane of light clockwise
o The levorotatory isomer rotates the plane anticlockwise
o An equimolecular mixture of two isomers (called a racemic
mixture) does not rotate the plane of light because the
dextrorotation cancels out the levorotation
o The specific rotation is a characteristic measurable property of
an optical isomer at a certain temperature, concentration and
wavelength of light
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Document Summary

Most organic molecules have more complex structures than most inorganic molecules. Carbon"s formation of covalent bonds rather than ionic bonds is the result of its electron configuration and its electronegativity value. Carbon"s ground state configuration is [he] 2s2 2p2. The loss of four electrons to form the c4+ Cation requires energy equal to the sum if the ie1 through ie4, the gain of four electrons to form the c4- anion requires the sum of ea1 through ea4, the last three steps of which are endothermic. Lying at the center of period 2, carbon has an electronegativity, En=2. 5, that is midway between that of the most metallic element and the most non-metallic element. The number and strength of carbon"s bond lead to its property of catenation. Catenation is the ability to bond to itself. Carbon"s small size and ability to form hybrid orbitals and multiple bonds increase the number of different molecules by affecting molecular shape.

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